Mono-n-Nonyl Phthalate-3,4,5,6-d4 is the deuterium-labeled (d4) analogue of mono-n-nonyl phthalate, provided as an analytical reference standard for mass spectrometry, isotopic tracing, and method development. It is intended for use as an internal standard in quantitative LC-MS/MS assays and environmental or biological phthalate analyses.